Positive power flow from the engine to the opposite ground-engaging members of a vehicle during steering is particularly desirable over those systems that apply power to one side while interrupting the power flow to the other side and subsequently applying the service brake at the other side. Rather than providing duplicate pump and motors for each side of the vehicle and the relatively complex control systems associated therewith, it is also preferred that a single pump and a single motor be utilized for steering to reduce manufacturing and service costs.
U.S. Pat. No. 4,434,680 issued to C. W. Riediger, et al on Mar. 6, 1984 and U.S. Pat. No. 4,541,497 issued to C. W. Riediger, et al on Sept. 17, 1985, respectively illustrate a particularly effective planetary steering differential employing a single steering pump and single steering motor, and a mechanical-over-hydraulic control system for the operation thereof. Steering of the vehicle is achieved by manual manipulation of a tiller arm that is oriented in a generally transverse disposition forwardly of the left arm rest of the operator seat. One disadvantage of such an arrangement is that the operation of the tiller arm is different from the operation of the usual automative-type steering wheel. Another disadvantage of that arrangement is that the vehicle does not travel on the same path upon reversing the direction of movement of the vehicle during a turn. For example, should the vehicle be making a turn to the right while proceeding forwardly the vehicle will not travel over the same path if the vehicle is shifted into a reverse mode of operation, but rather will deviate therefrom since the vehicle continues to rotate about its own central vertical axis at substantially the same turning rate. That mode of operation has a number of advantages, but can be initially perplexing to those operators familiar with only automotive-type steering systems that traverse over the same path in reverse.
Still another feature of the steering control system shown in U.S. Pat. No. 4,541,497 is that it incorporates an open-loop hydraulic system between the steering pump and the steering motor along with a single steering selector spool to operate it. In other words, in that system, the steering pump furnishes fluid to the steering circuit and also to other circuits such as to an implement circuit. This is disadvantageous from the standpoint that a problem with the implement circuit, such as contamination by dirt during servicing thereof, could detrimentally affect operation of the steering system. In view of this, a fully dedicated steering system is considered desirable for increased reliability. This is especially so if the vehicle to be used is operated at faster ground speeds and on public highways or the like.
A fully dedicated steering system using a conventional over-center steering pump requires a reliable method of adjusting the pump displacement control. The pump displacement control is normally associated with adjusting the angle of the swash plate of the pump. This can be accomplished solely by a mechanical connection between the steering wheel and the pump displacement control, but it is difficult to provide the desired degree of response and sensitivity with a mechanical connection. For example, each connecting joint requires a certain amount of clearance which increases with wear, and this contributes to an undesirably large steering wheel deadband. Then there is the matter of obtaining the desired steering wheel modulation and force required to turn the steering wheel through its maximum turning stage.
Accordingly, what is needed is a simple, safe and reliable vehicle steering apparatus that will be responsive to the rotation of an input member such as a steering wheel to more effectively actuate the pump displacement control of an over-center steering pump through a hydraulic system. It is also desirable to avoid using high pressure and large flow valve assemblies because of the expense thereof and waste of energy. Such a steering apparatus must provide a mechanical force sufficient to automatically return the input control member to a straight ahead or centered position, a reasonable steering force range for the associated steering ratio, a relatively fast rate of response, and a relatively narrow steering wheel deadband at the centered position. Preferably, the steering apparatus should be compatible with a closed loop steering pump and motor system and provide torque in a single power path to a differential mechanism of the type providing powered turning to both sides. And still further, the steering apparatus should preferably provide an automotive-type of capability wherein when the transmission is shifted into a reverse mode of operation during a turn, the vehicle will travel over the same path as in forward.
The present invention is directed to overcoming one or more of the problems set forth above.